1. Field of the Invention
The present invention relates to an ink ejector for forming an image on a recording medium such as paper by ejecting ink in accordance with print instructions.
2. Description of Related Art
Ink jet printers have a simpler principle than any other non-impact printers, and are easy of multiple gradation and colorization. Drop-on-demand ink jet printers eject only droplets of ink for printing. Ink jet printers of this type are coming rapidly into wide use because of high ejection efficiency and low running costs.
For example, U.S. Pat. No. 4,879,568, U.S. Pat. No. 4,887,100, U.S. Pat. No. 4,992,808, U.S. Pat. No. 5,003,679 and U.S. Pat. No. 5,028,936, which correspond to Japanese Patent Application Laid-Open No. 63-247051, disclose ink ejectors of the shear mode type as drop-on-demand inkjet printers. Each of the ejectors includes a controller and an ink jet head. The head has actuator walls of piezo-electric material, which are arranged in pairs to define channels between them. The head also has nozzles for the respective channels.
A conventional ink ejector of this type ejects a series of ink droplets in accordance with the print instruction for one dot. The ejector ejects each of the droplets by generating a pressure wave vibration in the appropriate ink channel. After ejecting the serial droplets, the ejector substantially cancels the pressure wave vibration in the channel by once increasing the volume of the channel and then decreasing it. In other words, the ejector carries out a cycle of ejection of serial ink droplets and cancellation of vibration in accordance with the print instruction for one dot. The number of such cycles per second is the drive frequency of the ejector.
This ejector can form a thick image on paper or another recording medium, and prevents the residual pressure wave vibration in the channel from producing ill or adverse effects on the next ejection.
After the droplets are ejected, the pressure wave vibration in the channel is very complicated. It is therefore very difficult to completely cancel this vibration. For this reason, the cancellation of vibration after ejection of serial ink droplets requires very accurate timing.
The ink jet head of this ejector is mounted on a carriage, which can be driven by a motor. While the carriage is moving along a recording medium, a print instruction signal is generated for each dot on the basis of the associated positional signal from an encoder. The resistance to the carriage movement may not be uniform, and the rotational speed of the carriage motor may fluctuate. There may be a case where the drive frequency fluctuates by about xc2x15%. In this case, as compared with a case where the drive frequency is constant, the cancellation of vibration for a dot may not occur at the predetermined time with respect to the pressure fluctuation caused for the preceding dot in the appropriate channel. As a result, the cancellation may not be effective, and the droplets may differ in volume by about xc2x115%. This may worsen the printing quality. In particular, this may make the quality very poor in a case where ink is ejected at a uniform gradation in a wide area. In order to keep the drive frequency constant, the fluctuation of the moving speed of the carriage could be restrained. This would require an expensive motor, an expensive driving system and a precisely machined or wrought carriage guide, resulting in greatly increased part costs.
It is an object of the invention to provide an ink ejector for ejecting a series of ink droplets to print one dot, and for always good printing quality without need for high part costs. It is another object of the invention to provide an ink jet printer having a ink jet head for ejecting a series of ink droplets to print one dot, which can keep good printing quality without need for high part costs.
In accordance with a first aspect of the invention, an ink ejector is provided, which includes an ink jet head for ejecting ink. The head has an ink channel formed therein, which can be filled with ink. The head also has an ink nozzle formed therein and communicating with the channel. The head includes an actuator provided therein for changing the volume of the channel. The ejector also includes a controller for controlling the actuator to change the volume of the channel to carry out ejection of an ink droplet from the channel through the nozzle a plurality of times in accordance with a print instruction for one dot. The controller controls the actuator to carry out substantial cancellation of pressure wave vibration in the channel after each ejection is carried out.
Thus, after ejecting each ink droplet, the ejector substantially cancels the pressure wave vibration to stabilize the pressure in the ink channel. This greatly simplifies the vibration which should be substantially canceled after each droplet is ejected. Therefore, even if the ejection and the substantial cancellation deviate slightly from the optimum points of time, it is possible to substantially cancel the vibration without difficulty. Consequently, even if the drive frequency fluctuates slightly, it is possible to eject a substantially constant amount of ink at each of the times. It is accordingly possible to maintain good printing quality without raising part costs by using an expensive motor etc.
The substantial cancellation may involve increasing the volume of the ink channel and decreasing the increased volume after a time WS. The inventor made experiments with the time WS varied, and a study of conditions for most effectively canceling the pressure wave vibration in the channel. The experiment and study revealed that the time WS should be set between 0.3T and 0.7T or between 1.3T and 1.8T where T is the one-way propagation time when a pressure wave of ink propagates one way in the channel. The reason for this is presumed as follows.
If the time WS is approximately 1.0 T (WS≈1.0 T), the peak of the pressure wave vibration generated by the increase in volume of the ink channel and the rise in pressure due to the decrease in volume of the channel (at point SE1) are superimposed on each other, ejecting ink from the channel. If the time WS is approximately 2.0T (WS ≈2.0T), the pressure wave vibration due to the increase in volume and the pressure wave vibration due to the decrease in volume cancel each other. The result of this case is similar to that of a case where no cancellation is carried out. It is therefore conceivable that good substantial cancellation of pressure wave vibration can be carried out with the time WS set between 0.3T and 0.7T which are intermediate between 0 and 1.0T or between 1.3T and 1.8T, which are intermediate between 1.0T and 2.0T.
The ejection and the substantial cancellation may involve applying an ejection voltage pulse of predetermined timing and width and a cancellation voltage pulse of predetermined timing and width, respectively, to the actuator. The ejection voltage pulse may have a width which is an odd number of times as long as T. The cancellation voltage pulse may have a width between 0.3T and 0.7T or between 1.3T and 1.8T. The time between the trailing edge of the ejection voltage pulse and the middle point of the cancellation voltage pulse may range between 2.35T and 2.65T, and preferably be about 2.5T.
The substantial cancellation may involve increasing the volume of the ink channel once when the pressure in the channel is high, and restoring the increased volume. Otherwise, this cancellation may involve decreasing the channel volume once when the pressure in the channel is low, and restoring the decreased volume. If the ejection involves increasing the channel volume once and restoring the increased volume, it is preferable that the substantial cancellation likewise involve increasing the volume once and restoring the increased volume. In this case, because the actuator is driven in the same directions for the ejection and the substantial cancellation, it is possible to simplify the drive means etc. in structure.
The ink ejector may also include a carriage for carrying the ink jet head thereon by moving along a surface of a recording medium. The ejector may further include a detector for detecting the position of the carriage. On the basis of the detected carriage position, the controller controls the actuator. The detector may include an encoder extending in the directions in which the carriage can move. The detector may also include a sensor fitted on the carriage. The sensor can read the indexes of the encoder.
In accordance with a second aspect of the invention, an ink jet printer is provided, which includes an ink jet head for ejecting ink. The head has an ink channel formed therein, which can be filled with ink. The head also has an ink nozzle formed therein and communicating with the channel. The head includes an actuator provided therein for changing the volume of the channel. The head is mounted on a carriage for moving along a surface of a recording medium. The position of the carriage can be detected by a detector. On the basis of the detected carriage position, a controller controls the actuator to change the volume of the channel to carry out ejection of an ink droplet from the channel through the nozzle a plurality of times in accordance with a print instruction for one dot. The controller controls the actuator to carry out substantial cancellation of pressure wave vibration in the channel after each ejection is carried out.
Thus, the actuator of this printer is controlled on the basis of the detected position of the ink jet head. Specifically, on the basis of the detected position of the head or the carriage, the ejection of ink from the head for each dot is timed. This ensures proper ejection timing even if the rotational speed of the motor driving the carriage, on which the head is mounted, fluctuates, or even if a drive mechanism such as the pulleys and the belt which transmit the driving force of the motor to the carriage slips. In the meantime, the actuator drive frequency fluctuates as stated above. Specifically, the ejection for dots does not start at a constant frequency. Accordingly, for example, when ink is ejected for a certain dot after ink is ejected for the preceding dot, the substantial cancellation for the certain dot is carried out at a point different from the predetermined point with respect to the pressure wave generated by the ejection and the substantial cancellation for the preceding dot.
The printer according to the invention carries out the substantial cancellation after the ejection of each droplet. Therefore, the cancellation timing is simple. This reduces the bad influence of the fluctuation in the amount of ejected ink, the fluctuation occurring if the actuator drive frequency fluctuates. Consequently, even if the frequency fluctuates, the amount of ejected ink per dot can be kept substantially constant. Therefore, this printer can eject ink at equal gradation for good printing quality, and needs no expensive motor and/or no accurate or precise carriage guide for keeping the drive frequency constant.
In accordance with a third aspect of the invention, another ink ejector is provided. This ejector includes an ink jet head for ejecting ink. The head has an ink channel and an ink nozzle formed therein. The channel communicates with the nozzle and can be filled with ink. The head includes an actuator provided therein for changing the volume of the channel. The ejector also includes a controller for controlling the actuator to change the volume of the channel to carry out an ejection operation which ejects an ink droplet from the channel through the nozzle xe2x80x9cNxe2x80x9d times in accordance with a print instruction for one dot. The number xe2x80x9cNxe2x80x9d is an integer which is equal to or more than two (2xe2x89xa6N). The controller so controls the actuator as to carry out at least twice a combination of xe2x80x9cnxe2x80x9d ejection operations, each of which ejects an ink droplet from the channel through the nozzle and a cancellation operation which substantially cancels pressure wave vibration in the channel subsequent to the ejection. The number xe2x80x9cnxe2x80x9d is an integer which is equal to or more than one, but smaller than the number xe2x80x9cNxe2x80x9d (1xe2x89xa6nxe2x89xa6N).
Every time this ink ejector has performed at least one ejection operation for ejecting an ink droplet from the channel, it substantially cancels the pressure wave vibration in the channel to stabilize the pressure in the channel. This relatively simplifies the pressure wave vibration which should be canceled each time by the cancellation operation. Therefore, even if the ejection operations and the cancellation operation deviate slightly from the optimum points of time, it is possible to substantially cancel the pressure wave vibration without difficulty.
The ink ejector may carry out, at least twice, the combination of two or three ejection operations and the cancellation operation subsequent to the ejection operations. Otherwise, the ejector may carry out, at least twice, the combination of one ejection operation and the cancellation operation subsequent to the one ejection operation. If the ejector carries out the cancellation operation every time it has performed two or more ejection operation, it is possible to shorten the time taken to eject ink a number of times for one dot. This results in high speed recording.
In accordance with a fourth aspect of the invention, another ink jet printer is provided. This printer includes an ink jet head for ejecting ink. The head has an ink channel and an ink nozzle formed therein. The channel communicates with the nozzle and can be filled with ink. The head includes an actuator provided therein for changing the volume of the channel. The head is supported by a carriage which can move along a surface of a recording medium. The printer also includes a detector for detecting the position of the carriage. The printer further includes a controller for so controlling the actuator on the basis of the detected carriage position to change the volume of the channel to carry out ejection of an ink droplet from the channel through the nozzle xe2x80x9cNxe2x80x9d times in accordance with a print instruction for one dot. The number xe2x80x9cNxe2x80x9d is an integer which is equal to or more than two (2xe2x89xa6N). The controller so controls the actuator as to carry out at least twice a combination of xe2x80x9cnxe2x80x9d ejection operations, each of which ejects an ink droplet from the channel through the nozzle and a cancellation operation which substantially cancels pressure wave vibration in the channel subsequent to the ejection. The number xe2x80x9cnxe2x80x9d is an integer which is equal to or more than one, but smaller than the number xe2x80x9cNxe2x80x9d (1xe2x89xa6n less than N).
In this ink jet printer, as stated above, the pattern of ejection operations and cancellation operation is adjusted. This makes it possible to effectively cancel the pressure wave vibration, even if the rotational speed of the motor for driving the carriage fluctuates, or a drive mechanism such as the pulleys or the belt for transmitting the driving force of the motor to the carriage slips, causing the actuator drive frequency to fluctuate. Therefore, this printer can eject ink at equal gradation for good printing quality, and needs no expensive motor and/or no accurate carriage guide for keeping the drive frequency constant.